1. Field of the Invention
The invention relates to a method and a device for a full-duplex-capable radio transmission system with DS-CDMA access, having a central radio base station and a plurality of subscriber stations which are independent of one another.
2. Discussion of the Prior Art
In the field of radio-supported information systems which operate with a central radio base station and a plurality of external stations or subscriber stations which are independent of one another and which permit information to be transmitted in full duplex form in both directions, the information in the downlink which is intended for the individual users is frequently multiplexed into a telecommunications channel and transmitted organized as an access system in the uplink. Examples of such systems are mobile radio systems, public trunked mobile radio systems, point-to-multipoint microwave radio systems and wireless local loop systems. Orthogonal signal domains which differ from one another are used in each case for the multiplexing or multiple access, these signal domains being, for example,
frequency division multiplex or access systems FDMA (frequency division multiple access)
time division multiplex or access system TDMA (time division multiple access)
code division multiplex or access systems CDMA (code division multiple access) or SSMA (spread spectrum multiple access), and
space division multiplex or access systems.
The systems differ in that the transmission of information from and to the individual users takes place in separate frequency, time, code or spatial segment positions. Interleaved, coupled or respectively different multiplex and access technologies within one system, so-called hybrid methods, have also become known. Depending on the use and implementation, different transmission parameters and transmission quality criteria can be obtained with these methods.
In CDMA systems, the user signal is coded by gating it with a spread function using logic operations, a separate spread function which is orthogonal to the other spread functions being selected for each subscriber station. The logic operation is carried out here in each case by means of an X-OR gate, for example. At the receiver end, the coded signal can be demodulated through knowledge of the associated spread function, the coded user data for other subscriber stations becoming zero during the demodulation process owing to the orthogonality. It is particularly advantageous with CDMA systems that all the users can operate in the same frequency band and a relatively high degree of interference power in the band can be tolerated. Furthermore, under certain conditions it is possible that adjacent radio cells can operate on the same frequency band. It is generally a disadvantage that the multi-user interference, which arises in practice as a result of implementation problems such as band limitation, level differences between the individual transmissions, multi-path propagation etc. and which leads to a loss of orthogonality. In the radio systems under consideration, it is to be noted basically that because of the different signal transit times owing to different distances between the external stations and the central station an asynchronous reception situation is normally produced in the base station receiver, which situation considerably aggravates this interference to such an extent that under ideal conditions code orthogonality is then no longer produced in the uplink. In this case, the maximum number of simultaneous transmissions M within a frequency band in the uplink of, by approximation, a DS-CDMA system can be estimated as follows:
xe2x80x83M=PG/(Eb/No),
PG being the process gain or spread factor and Eb/No being the ratio of bit energy to interference power, necessary for the aimed-at bit error rate, at the demodulator. The spread factor is the ratio of tbit to tchip and is typically between 101 and 104.
Assuming the ratio Eb/N0 is, for example, 3, which corresponds to approximately 5 dB, in terms of the same bandwidth being seized, only approximately ⅓ of the transmission capacity is available in the uplink in comparison with the downlink or in comparison with TDMA or FDMA systems if orthogonal signals are assumed for the latter.
Various methods are known which reduce the previously described disadvantage of the asynchronous CDMA methods, for example the synchronization of the external station in such a way that its transmission can be processed chip-synchronously in the receiver of the base station. In addition, it has been proposed to implement interference cancelers which, by means of mathematical algorithms, subsequently eliminate the interference component of the parallel transmissions on the basis of different a priori or a posteriori knowledge. Furthermore, it has also been proposed to use multi-user detectors. A disadvantage of all these known methods is that they are very costly to implement.
The invention is therefore based on the technical problem of providing a method and a device for a full-duplex-capable radio transmission system with DS-CDMA access, in which the ratio of uplink transmission capacity with respect to downlink transmission capacity is improved.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in dividing the reception antenna in its horizontal antenna characteristic into a plurality of reception segments which are each connected to an evaluation unit by means of which the reception segment with the best reception quality can be determined on a subscriber-specific basis and can be selected for the rest of the data processing. In this way the influence of interference of other, simultaneously transmitting subscriber stations is considerably reduced, since it is possible to assume that all the subscribers are separated from one another with approximately uniform spatial distribution in the region of influence of the base station reception antenna segments, and there is also no marked correlation between the subscriber location and parallel transmission. Consequently, the quality of the separation of the individual reception signals depends essentially on what transfer occurs as a result of the specific characteristic of the antennas, what interference occurs in the regions of the other respective antenna apertures as a result of multipath propagation and how many subscribers are transmitting simultaneously, considered statistically and instantaneously, in the spatial segment of an antenna.